In recent years, video cameras and digital still cameras employing solid-state imaging device such as CCDs (Charged Coupled Devices) and CMOS (Complementary Metal-Oxide Semiconductor) devices have been in widespread use. Such image pickup apparatus are required to achieve a higher image quality. In particular, digital still cameras with a large number of pixels need imaging lenses having an excellent focusing capability compatible with solid-state imaging devices with a large number of pixels, especially zoom lenses having a high variable magnifying power. In addition, low-profile zoom lenses are particularly needed to meet strong demands for smaller image sensing devices.
In view of the above needs, zoom lenses disclosed in Japanese Patent Laid-open No. Hei 8-248318 and Japanese Patent Laid-open No. 2000-131610 are designed to reduce the size of a front lens in the direction of the optical axis, i.e., to achieve a low-profile lens design, by inserting a prism in an optical system.
However, the zoom lenses disclosed in Japanese Patent Laid-open No. Hei 8-248318 and Japanese Patent Laid-open No. 2000-131610 are problematic in that it is difficult for the zoom lenses to have a higher magnifying power because the higher magnifying power tends to result in greater aberration changes when the magnifying power varies. Furthermore, even if the zoom lenses have a higher magnifying power, since the movable lens groups have a positive refractive power, the movable lens groups have a large mechanism, which makes it difficult to make the zoom lenses lower in profile. Specifically, the zoom lenses disclosed in Japanese Patent Laid-open No. Hei 8-248318 and Japanese Patent Laid-open No. 2000-131610 have second and fourth lens groups movable for zooming. Inasmuch as the fourth lens group of each of the zoom lenses has a positive refractive power, the movable lens groups include the lens group having a positive refractive power, making it difficult to make the zoom lens smaller.
The present invention has been made in view of the above problems. It is an object of the present invention to provide a zoom lens which has good optical performance, is compact, and has a high magnifying power, and which is suitable for use in a video camera, a digital still camera, a cellular phone, or the like, and an image pickup apparatus employing the zoom lens.